Selectivity control



Aug. 20, 1940 R. c. CORDERMAN SELECTIVITY CONTROL Filed Aug. 3, 1938 IU hl... NURDQQ msm wJmSN() 0.-. W

Lul d lum w I l QN L J'N w Saz. NN @N D muu 9. m+ zummum l. A @JN Roy C Corclernuub @www PatentedAug. 20, 1940' IJlSuTlezD STATES y 2,212,391 r sELEC'rIvrrY CONTROL Roy 0. Corderman, Chevy Chase, Md.

Application August 3,

v 10 claims.

This invention relates to radio and allied electrical apparatus, and more particularly to a method and means for effecting avariation' in vthe resonance curve of a Vtuned circuit, or circuits,

"in such apparatus whereby selectivity, and consequently the fidelity of program .reproduction may be controlled.

In radio reception it frequently occurs that the desired ksignal is partially blanketed by signals adjacent inl frequency which are receivedV with equal or greater strength thanthe desired signal. If the receiver can lbe made more selective, the response to, the undesired signals is proportionately lessened, and likewise to some degree with extraneous noise. When the desired signal is free from adjacent channel `interference it is desirable to `dispense with extreme selectivity in order that the modulation sidebands of the signal may be accommodated and the full range of 'higher audio frequencies occurring in the modulation may be reproduced. i

Variable selectivity may be achieved by mechanical methods of changing transformer coupling or by various electrical means such as the use of a positive or a negative feedback, or the use of a tube as a variable impedance across the plate circuit'of an amplifier tube. vThe electrical methods mentioned are subject to control by the `acm tion of the automatic volume control system of' a receiver and automatic selectivity is accomplished. When a signal is weak and hence is most subject to interference, the automatic volume control bias is reduced in order to effect greater amplification. This reduction` of bias is utilized to increase the selectivity of the receiver conjointly with the increase in amplification. In thev case of-a strong signal', the bias goes more negative,

the amplication of the receiver is reduced, andl the response curve of the receiver is broadened 40 to permit full range audio reproduction.

It is a primary object of the present invention to provide an improved method of electrically 'controlling the selectivity of an amplifier stage in radio and related apparatus. f

5 5`r kIn its present preferred form, the invention-'vis' Another object is to provide a system of selec-f tivity control which functions automatically in 1938, f senat No. 222,907

described and explained as embodied in a receiver of the superheterodyne, type', and is utilized in connection with the intermediate frequency ampliier` of the receiver. Briey summarized, the

invention contemplates a piezo-electric crystal 8 which lis ground to resonate at the intermediate frequency, andwhich is connected between the cathode and a grid element of the amplifier tube. As the transconductance of the tube is varied by y means 'of grid bias, the 'selectivity of the input circuit is varied. A low value of negative bias causes maximum selectivity and a progressive reduction of selectivity occursas-the value of negative bias is increased. It will be obvious that any suitable 'source of bias may be employed and the bias potential may be controlled manually as by a potentiometer, or the grid element or elements to be controlled may be connected to the automatic volumecontrol system vof anlassociated rev ceiver and obtain bias potentials automatically `regulated in value from that source.

The accompanying drawing and following description of the invention are intended only for purposes of illustration and the invention is not to be considered as limited to the particular modes of application and function to r be later set forth in detail. Additional objects and. advantageswill in part be presented specifically and in vpart be obvious when the description is read iny connection withthe drawing, in which:

Figure 1 is a circuit diagram, partly schematic, of a complete superheterodyne receiver embodying a preferred form of the present invention; and Figures 2 and 3 are alternative modes of convnection of the filter circuit schematically presented in Figure. 1. f

Referring now in more detail to Figure l, there is shown diagrammatically a complete superheterodyne receiver in 'which a tunable radio frequency amplier stage i'isr connected to an antenna 2. A modulator'stage 3 receives the output of theradio frequency amplifier l and of an` oscillator t, and produces a modulated wave of the. desired intermediate frequency in the output circuit 5, which may comprise the primary of a conventional intermediate frequency transformer indicated generally as (i.rr The secondary winding 1 of the transformer is included in the input circuit of "a multi-grid-ampliiier tube v8, whichin this particular embodiment is yof the type commercially designated as .a SLT. The intermediate frequency signal is applied between the cathode S and signalfgrid I0 of the tube. A screen grid il is provided; with a suitable value of positive potential and isv situated between the {if}vv signal grid It and what normally is the injectorgrid i2. The balance of the tube elements comprise a suppressor grid lliwhich is'connected to the cathode 9, andra plate I4 which is furnished l to the cathode 9 by means of the drop existing across the cathode bias resistance i8 and the circuit extending from ground through a high impedance i9, which is diagrammatically illusl trated as a radio frequency choke. It is. possible to substitute a high value of'resistance for the choke i9 without adversely affecting the circuit performance.

The output of the tube 8 is passed through anotherintermediate frequency transformer, indicated generally as 2t, andis applied to a second detector and automatic volume control stageZl of well known design and construction. The audio component of the rectied signal is further am-v plied by an audio amplifier 22 and is reproduced by a loud speaker 23. The rectified and filtered carrier component of the signal forms the automatic volume control bias, which is a negative potential varying in value ywith the strength of the received signal.l This bias is applied through lead 2t to the signal grid it of the amplifier tube i3, and to the signal grid of the tube included in the radio frequency amplifier stage i.

In the operation of the invention, the maximum selectivity is attained when thel bias on signal grid it is at the limiting lower value determined by the drop across the cathode bias resistor i8, which is to say, when a weak signal is being received.

' of the variable selectivity circuit illustrated in y u manually controlled potentiometer.

` trated in Figure 3, wherein like parts also bear Stronger signals will produce an automatic vol` ume control bias which isin excess vof this lower limit and selectivity will be lessened proportionately. When strong signals are being received as from a local broadcasttransmtter or other `powerful station, thejselectivity is reduced to substantially that of the circuit in the absence ofthe piezo-electric `crystal il.

In Figure 2 there is shown an alternative form Figure 1, and like parts'therein bear like reference numerals. The form shown in Figure '2 has the negative bias on the signal grid l@ held to the lov.T value determined by the voltage drop occurring across the cathode bias resistor i8, while the bias on the grid l2 is varied to control the selectivity of the stage. The bias may be derived either from the automatic volume control system or fromy a voltage divider network and applied through a The operation of this form of theinvention differs somewhat from that shown in Figure 1 in that the selectivity will remain fairly high up to the middle ranges of bias applied to the gridi2, and the curve willthen expand more rapidly as thehigher ranges of bias are applied.

A further modification of theinvention is illusthesame reference numerals as in Figure 1. rIhe grids it and i2 in this modification are negatively biased in like degree from the automatic volume control system as shown, or from some other source of-variable biasing potential.

`It appears that inall instancesY the change of hence the'tube transconductance is large.

selectivity with changes of bias values are due to the accompanying changes of tube transconducf tance under the influence of varying potentials on the grids i0 and l2, either singly as in Figures 1 and 2, or jointly as in Figure- 3. In general, high selectivity occurs only when' the two grids are held to a relatively low value of bias, and

imposing unequal rates of change of bias on the grids it and i2, thefslope and rate of expansion of the selectivity curve may bevaried, and mayv even be caused to reverse over a portion of the range. f

It will be apparent that the invention herein shownand described may be applied toV many uses other than in connection with radio receivers,v and may havel each variable element individually and manually controlled or automatically controlled. While there have been illustrated what are` at present considered to be the preferred embodiments of the invention, many changes and modications may be made therein by anyone skilled 4in the art without departing from the spirit and' scope of the invention Vas expressed in the following claims. p

What is claimed is:

i. In a wave transmission system, an amplifier stage adapted to operate at a predetermined frequency, an electron tube in said stage, said tube including a cathode,a signal grid element, and a second grid element, a piezo-electric crystal resonant at said predetermined frequency, means connecting said crystal between said cathode and said second grid, means for maintaining a constant biasing potential on one of said grid'elements, andy means for applying a variable biasing potential to the other of said grid elements in response toy variations of signal strength in said system, whereby the selectivity of said amplifier stage may be varied. y

2. In a radio receiver having an automatic volme control system,l an amplier stage adapted to operate ata predetermined frequency, an elecgridQand means acting in response to said autoM l matic volume control for varyingv the transcon-v ductance of said tube, whereby ythe `overall selectivity of said receiver may be varied.

3. In a radio receiver lhaving an automati volume control system, an amplifier stage adapted to operate at a predetermined frequency, an electron tube in said stage,' saidtube having a cathode, a signalgrid element, Aand a second grid element, a piezo-electric crystal resonant at said predetermined frequency, means connecting said crystal between said cathode and said second grid, and a connection between one of said grids and said automatic volume control system, where-l by the overall selectivity of said receiver maybe varied in response to variations lin received signal strength.

4. In a radio receiver havingv anV automaticl volume control system," an `amplifier stage adapted to operate at a predetermined frequency,

an'electi'on tube in said stage, said tube having a cathode, ar signal grid element, and a second grid element, a piezo-electric crystal resonant at said predetermined frequency, means connectingsaid crystal between said cathodel and `said sec-1 ond grid, anda connection between said signall grid andr said automatic volume control system,

whereby the overall selectivity of said receiver u may be varied in response to variationsy in received signal'strength.

6. In a wave transmission system, an amplifier' stage adapted to operate at a predeterminedfrev quency, an electron tube in said stage, said tube including a cathode, a signal grid element, and a second grid element, a piezo-electric crystal resonant at said predetermined frequency, a connection between said crystal and said cathode, a second connection comprising a continuous conductive path between said crystaland` said second grid, and meansI for varying the transconductance of said tube, whereby the selectivity of said amplifier stage may be varied.

7. In a wave transmission system, an amplifier stage adapted to operate at a predetermined frequency, an electron tube in said stage, said tube including a cathode, a signal grid element, and a second grid element, apiezo-electric crystal resonant at said predetermined frequency, a connection between said crystal and said cathode, a second connection comprising a continuous conductive path between said crystaland said second grid, and means for varying the transconductance of said tube in response to variations of signal strength in said system, whereby the selectivity of said amplifier stage may b'evaried.

8. In a wave transmission system, an amplifier stage adapted to operate at a predetermined 'frequency, an electron tube in said stage, said tube including a cathode, a signal grid element, and a second grid element, a piezo-electricy crystal resonant at ,said predetermined frequency, a connection between said crystal and said cathode, a second connection comprising a continuous conductive path between said crystal and said second grid, and means for applying a variable biasing potential to one of said grid elements, whereby the selectivity of said amplifier stage may be varied.

9. In a wave transmission system, an amplifier stage adapted to operate at a predetermined frequency, an electron tube in said stage, said tube including a cathode, a signal grid element, and a second grid element, a 'piezo-electric crystal resonant at said predetermined frequency, a connection between said crystal and said cathode, a

second connection comprising a continuous con-` ductive path between said crystal and said second grid, impedance means effectively in's'hunt with said crystal, and means for app-lying a variable biasing potential to one of said grid elements, whereby the selectivity of said amplifier stage may .be varied.

10. In a Wave transmission system, an amplier stage adapted to operate at a predetermined frequency, an electron .tube in said stage, said tube including a cathode, a signal grid element, and a second grid element, a piezo-electric crystal resonant at said predetermined frequency, a connection between said crystal and said cathode, a second connection comprising a continuous conductive path between said crystal and said second grid, means fcr maintaining a constant biasing potential on one of said grid elements, and means for applying 'a variable biasing potential to the other of said grid elements, whereby the selectivity of said amplifier stage may be Varied.

ROY C. CORDERMAN. 

